In recent years, IC technologies using organic semiconductor devices have attracted people's attention. The main appeals thereof are a simple production process steps (low cost), and compatibility with flexible substrates. From these advantages, the applications of organic semiconductor devices are expected as plastic circuits, electronic tags, drive circuits for displays, and the key devices for memories.
In general, an organic semiconductor device is constituted of a substrate, a gate electrode, a gate insulating film, a source electrode, a drain electrode, and an organic semiconductor film, and is utilized as a thin film field effect transistor (FET) and the like. When a voltage applied to the gate electrode (gate voltage: VG) is changed, charge of an interface between the gate insulating film and the organic semiconductor film becomes excessive or deficient, and the drain-source current value (Ids) flowing between the source electrode, the organic semiconductor, and the drain electrode changes to enable switching.
The term “organic semiconductor” as herein employed is intended to embrace those materials that contain carbon or an allotrope of carbon and have a mobility of charge carriers of at least 10−3 cm2/Vsec at room temperature (20° C.).
Over past 10 years, IC technologies using organic semiconductor thin film transistor (TFT) have been proposed. The major appeal of these circuits stems from expectation in the ease of processing and the compatibility with flexible substrates. These advantages are expected for the use in low-cost IC technologies suited to applications, such as smart cards, electronic tags, and displays.
Reports on the results of heretofore researches and developments include F. Garnier et al., Science, Vol. 265, pp. 1684–1686; H. Koezuka et al., Applied Physics Letters, Vol. 62 (15), pp. 1794–1796; H. Fuchigami et al., Applied Physics Letters, Vol. 63 (10), pp. 1372–1374; G. Horowitz et al., Applied Physics Journal, Vol. 70 (1), pp. 469–475; and G. Horowitz et al., Synthetic Metals, Vols. 42–43, pp. 1127–1130. Devices described in these references use polymers or oligomers as active materials, unlike amorphous silicon and polysilicon TFT structures developed in the initial stage. The devices are generally field effect transistors (FETs).
Organic polymer devices have significant advantages of simple processing and resultant low cost in comparison with inorganic semiconductor devices. In addition, since organic polymer devices have compatibility with polymer substrates, which excel in workability and plasticity, devices having flexible structures can be easily formed. In such cases, due to the compatibility of the coefficient of thermal expansion with the substrates, stress to the bonding interface is smaller than the stress to an inorganic semiconductor/polymer substrate.
Well-known organic compounds exhibiting characteristics as semiconductors, used in devices include low-molecular compounds such as phthalocyanines, and π-conjugated oligomers and π-conjugated high-molecular compounds such as polythiophene. These organic semiconductor compounds form band structures consisting of a valence band, a conduction band, and a forbidden band like inorganic semiconductors, and oxidization or reduction (doping) by chemical or physical means forms carriers for carrying electric charge. Such organic semiconductor compounds can be applied to various devices, and several reports have hitherto been published.
Circuits utilizing organic semiconductor devices have a subject of improvement of the integration density, and require fine patterning of organic semiconductor films and electrodes. However, since conventional producing methods using printing or optical patterning require much labor and cost for fabricating screens and metal masks, they are not suited to applications wherein various kinds of design circuits optimized for various uses are produced in small quantities at low cost.